A star with a strange chemistry comes from outside the city

Astronomers have discovered a star in the Milky Way with a chemical composition unlike any other star in our galaxy.

This chemical composition has been observed in a small number of stars of dwarf galaxies orbiting the Milky Way. This suggests that the star was part of a dwarf galaxy that merged with the Milky Way.

In the LAMOST (Multi-Object Optical Fiber Spectroscopic Telescope) survey data, researchers noted the star J1124 + 4535 for its unusual chemical composition. Early observations showed that J1124 + 4535, located in the constellation Ursa Major, had low abundance of some elements, such as magnesium. Follow-up observations made with the high-dispersion spectrograph on the Subaru telescope confirmed low levels of magnesium, but revealed comparatively high levels of europium. This is the first time that an item report of this type is observed in a Milky Way star.

Stars form from interstellar gas clouds. The component reports of the parent cloud confer an observable chemical signature to the stars formed in that cloud. Thus, the stars formed close to each other have similar elemental relationships. The composition of J1124 + 4535 does not correspond to any other star in the Milky Way, which indicates that it must be formed elsewhere.

Chemical signatures similar to J1124 + 4535 have been observed in some stars of dwarf galaxies orbiting the Milky Way. Models and simulations of galaxy evolution suggest that galaxies such as the Milky Way develop by absorbing neighboring dwarf galaxies. It is therefore logical that J1124 + 4535 was born in a now extinct dwarf galaxy that merged with the Milky Way.

Reference: "Evidence of the origin of accumulation of Halo stars with extreme refinement of the process-r", Qian-Fan Xing et al., April 29, 2019, Nature Astronomy [https://www.nature.com/articles/s41550-019-0764-5].

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